The coupling can be swapped without removing the whole box IIRC, the front housing comes away (you loose all the oil) and in it is the Viscous unit.
The oil in the viscous is sealed in, so unless there are any horroras inside the tranfer box then there is no reaosn to suspect it's any worse than a second hand unit, after all if it's second hand the car is came from wasn't worth keeping on the road, right
Here's a few ways of checking.
With the handbrake on and the car in neutral, jack up one front wheel.
With mine I can:
Just about undo the propshaft UJ against the stiffness of the Viscous unit, if I push quick the nuts come undone, if I push slow the propshaft turns.
Undo a wheel nut, agai if you push quick the viscous fluid does it's job and locks up, if I lean on the wheel wrench it turns, very,
very slowly. That's all you need to happen.
I have heard about Visocu units locking up, it's caused by the Thermo-vicous fluid inside overheateing and turning to a hard rubber like substance that jams the fins within the unit, but I've
never seen one, even in all the cars we used to break at my mate's place.
Any one need a transfer box, I have too many spare units here but they do hold their money, I've seen just the Viscous unit go for £150 on flea-bay :o